The growth in use of small electrically-powered devices has increased the demand for very small metal-air electrochemical cells. Metal-air cells have gained significant popularity because only the anode reaction material need be packaged in the cell. In combination, the cathode reaction material is oxygen, which is drawn from the surrounding ambient environment.
Such small cells are usually disc-like or pellet-like in appearance, and are about the size of garment buttons. These cells generally have diameters ranging from less than 5.8 millimeters to about 25 millimeters, and heights ranging from less than 2.0 millimeters up to about 15 millimeters. The small size of such cells, and the limited amount of electrochemically reactive material which can be contained in such small metal-air cells, result in a need for improving the efficiency and completeness of the electrochemical reactions, which are used in such cells for generating electrical energy, and for improving the fraction of the overall volume of such cell which can be occupied by the electroactive anode material.
Such metal-air cells take in atmospheric oxygen, and convert the oxygen to hydroxyl ions in the air cathode by interaction with aqueous alkaline electrolyte. The hydroxyl ions then migrate to the anode, where they cause the metal contained in the anode to oxidize. Usually the active anode material in such cells comprises zinc, although a variety of other operable anode materials are well known to those skilled in the art.
More particularly, the desired reaction in the air cathode of a metal-air cell involves the reduction of oxygen, the consumption of electrons, and the production of hydroxyl ions. The hydroxyl ions migrate through the aqueous alkaline electrolyte toward the anode, where oxidation occurs, forming zinc oxide.
In typical metal-air cells, air enters the cell through one or more air ports in the bottom the cathode can. The port or ports extend through the bottom wall of the cathode can, and may be immediately adjacent the cathode assembly, or may preferably be separated from the cathode assembly by an air reservoir, which is typically occupied by an air diffusion member.
In such arrangements, the port facilitates movement of air through the bottom of the cathode can and to the cathode assembly. At the cathode assembly, the oxygen in the air reacts with water as a chemically reactive participant in the electrochemical reaction of the cell, and thereby forms the hydroxyl ions.
Since the overall electrochemical capacity of any electrochemical cell is to some extent determined by the quantity of electrochemically reactive materials which can be loaded into the cell, it is important to maximize, in the cell, the size of the cavity which is devoted to containing the electrochemically reactive materials. In the case of a metal-air cell, contained reactive material is limited to the anode material.
In general, the size of any given cell is limited by the inside dimensions of the space provided in the article, namely the appliance, in which the cell will operate. For example, the size of a hearing aid cell is limited to the internal dimensions of the space, provided for the cell, in the hearing aid appliance. The internal dimensions of the space are determined by the hearing aid manufacturer, not the power cell manufacturer.
Thus, any given appliance includes a limited amount of gross space or volume allotted to occupancy by the electrochemical cell which powers the appliance. That gross space may ultimately be divided according to four functions, all competing for portions of the gross space. A first portion of the space is used to provide clearance between the interior elements of the space and the exterior elements of the electrochemical cell.
A second portion of the space is occupied by the structural and otherwise non-reactive elements of the electrochemical cell.
A third portion of the space is allocated for occupation by the electrochemically reactive material in the electrochemical cell, and, in a metal-air cell, especially the anode material.
Finally, a fourth portion of the space, as appropriate, can sometimes be described as "wasted" space, because it serves none of the above first through third functions. Such wasted space is typically found outside the cell, e.g. at corner locations, where the corner of the cell is less square than is structurally feasible, thereby wasting volume that potentially might be occupied, either directly or indirectly by electrochemically reactive material. Such wasted space might also be considered to be included in the space allocated to clearance because such space is typically located outside the cell.
Any increase in the third portion of the space, namely the cavity in the anode can which cavity is allocated to the anode material, is necessarily gained at the expense of one or more of the other three portions of the fixed volume allocated for occupation by the cell, namely the first clearance portion, the second portion devoted to the non-reactive elements of the cell, or any fourth waste portion. Thus, it is important to identify the first, second, and fourth portions of the overall space, and, where possible, to reduce the amount of space devoted to such uses. To the extent such uses can be reduced, the space so recovered can, in general, be allocated for use to hold additional amounts of electrochemically reactive anode material, thereby increasing the potential overall capacity of the cell to generate electrical energy within the limited amount of gross space or volume provided, in the appliance, for occupation by the cell.
Overall cell height and width dimensions are specified by the International Electrotechnical Commission (IEC).
Of the first, second, and fourth portions of the cell, the opportunity for capturing space from the first portion, devoted to clearance, relates in part to the ability of the manufacturer to control the range of outer diameters of the cathode cans from which the cells are made. To the extent the range of diameters can be reduced, nominal clearance may be reduced accordingly.
It is known that traditional methods of forming cathode cans for use in hearing aid cells have a tendency to form an outward bulge in the diameter of the cathode can at the intersection of the side wall with the bottom wall, whereby allowance must be made in the can specification for occurrence of such bulge.
In addition, applicants have concluded that further potential for recovering space for use in holding anode material, and thus to increase volume efficiency of the cell, lies primarily in the second portion of the cell, namely the structural and otherwise non-reactive elements of the cell. These elements generally comprise the cathode can, the anode can, the seal, and the cathode assembly, these typically representing all of the major structural elements of the cell. Thus, to get more space from the second portion of the cell, that space must be taken from the anode can, the cathode can, the cathode assembly, or the seal, or some combination of these.
This invention focuses on apparatus, methods, and materials for providing improved cathode cans, and wherein the cathode cans have reduced cross-section thicknesses, and reduced range of thicknesses from can to can, while maintaining suitable strength parameters to properly support the manufacture and use of such cans, and cells made therewith. Such cans typically have a pair of nickel layers, and a steel layer between the nickel layers.
It is known to desire to reduce the thickness of the non-reactive structural materials of the cell. However, the desire to reduce the thickness of such non-reactive elements operates in tension against the requirement that such structural elements have suitable fabrication properties, and suitable strength to support the fabrication and use of the cell. Accordingly, an element cannot simply be made thinner without considering the effect such thinning will have on the ability to fabricate the element, or to fabricate and use a cell made therewith.
Similarly, it is known to select different materials from which to fabricate the respective non-reactive elements. Such different materials may have different chemical composition, or different chemical or physical properties. However, changing material selection also affects the ability to fabricate the element, and the ability of the element to support fabrication and use of the cell.
Thus, where thinner, or harder, metal strip is contemplated for use to form cathode cans, there is the prospect of developing cracks in one or more of the layers of the metal strip during conventional fabrication of the can.
Accordingly, any change in selection of material from which the cans are to be made, or physical dimensions of such material, must be carefully balanced against the fabrication requirements associated with such material as the material is used to fabricate the respective elements; as well as the requirements associated with fabrication and use of a cell utilizing such elements. Any change of material must, of course, be compatible with the chemical operating environment within which the cell operates. Typically, air depolarized cells operate in an alkaline environment, and so any material used therein must be compatible with such environment.
It is an overall object of the invention to provide improved air depolarized electrochemical button cells.
It is a more specific object of the invention to provide cathode cans wherein the side walls are harder than the bottom walls.
It is yet another object to provide cathode cans wherein outwardly-disposed side walls have smoother finishes than corresponding surfaces of the respective bottom walls.
It is still another object to provide can forming systems including a punch and a die wherein the clearance between the punch and die is less than the thickness of a metal strip to be formed therebetween.
Yet another object is to provide methods of making cathode cans in a metal strip, wherein the clearance between the punch and the die is less than the thickness of the metal strip.
It is a further object to provide cathode cans wherein the side wall is harder than, and thinner than, the bottom wall, and wherein an outwardly-disposed surface of the side wall has a smoother finish than a corresponding outwardly-disposed surface of the bottom wall.